Size Effect On Strength And Energy Dissipation In Fracture Of Rock Under Impact Loads

Size effect is the most important aspect in all physical theories. The extensive research on the size effect of materials strength under static load has been carried out by many scholars, and a great progress has been made by now. However, it is essential to obtain the dynamic characteristics and to understand dynamic properties of rock-like materials in a lot of project fields and some fields of geological hazard prevention and control. It can be said that the dynamic mechanical properties of materials is as important as the static characteristics. While there are seldom reports about the size effect on the dynamic mechanical properties of materials. Therefore, the author of this thesis carried out the research deeply and systematically on the size effect of rock strength and energy dissipation in fracture under impact loads with the support of the National Natural Science Foundation of China.Firstly, A serie of SHPB testing equipments with different elastic bar's diameter of 22mm, 36mm 50mm and 75mm were improved or developed. Striker series which can realize a half-sine loading waveform in the input bar were designed and processed corresponding to the SHPB equipments. Based on Visual C++ language, improvement had been made on the existing data processing software used for SHPB tests with elastic bar's diameter of 50mm and 75mm. The function of main interface and the identification ability of the software on the time sampling interval of the collected signals were added. Aiming at the problem of noise in signal of SHPB tests, considering the characteristics of non-stationary, short duration and abrupt change in signal structure, it was determined that the Sym5 in Symlets wavelet series was the optimal wavelet basis suitable for signal analysis. The Gauss White Noise in signal can be removed effectively using threshold de-noising method based on wavelet transform.Secondly, based on the characteristics of one-dimensional waves, the specimens stress uniformity process under different loading conditions of rectangle and half-sine input waves was analyzed furtherly in Split Hopkinson Pressure bars (SHPB) tests from the two aspects: the time required for stress uniformity and the value of dimensionless stress corresponding to the uniform stress state in a specimen. It was confirmed that the half-sine pulse loading method has significant advantages in the dynamic measurement of rock-like materials with low strength.Thirdly, based on the brittle fracture criterion of Steverding-Lehnigk, the relationship between SHPB diameter and minimum loading rate causing rock failure under half-sine input waves loading was analyzed and obtained, and validated by SHPB tests using granite, sandstone and limestone specimens with different sizes. It was demonstrated that increasing the diameter of Hopkinson bar is one of the effective approaches to realize the rock medium strain rate loading.Fourthly, using the loading method of half-sine stress waveform with wave length proportioned to the specimens' length, a series of SHPB tests under different strain rates were carried out on granite, sandstone and limestone specimens with different sizes. The size effect of rock strength and strain rate sensitivity was investigated. Considering of the inertial effects of the medium, the analysis calculation was carried out using FLAC3D on geometrically similar cylinder specimens of different sizes subjected to axial impact.Fifthly, the relationship between the incident energy, the absorption energy of specimen, the density of energy consumption and the fragment size distribution, and the effects of specimen size on them were revealed by experiments.Finally, simplifying the stress conditions of specimen in SHPB tests, the relationship between dynamic fracture toughness and strain rate, energy compositions and the characteristics of the energy dissipation of rock specimen with different sizes were simulated according to the sliding crack model and dynamic crack growth criterion.In short, the research of this dissertation, which is based on the frontal of the subject, is mainly concerned with the impact loading test technique of SHPB and the size effect on the dynamic properties of rock. It is useful for both theory and practice.